Automobile and light truck bodies and structural elements are commonly formed from sheet metal components which may range in thickness from about 0.65 millimeter for outer body panels and up to about 6 millimeters for frame rails. Each component will comprise combinations of features, such as depressions, radii etc. as dictated by structural or aesthetic considerations, or both. These components are generally shaped by stamping, that is, an incoming flat sheet obtained from a supplier is placed between a pair of dies of complementary shape and the dies are closed on the sheet to impart the desired shape to the sheet.
The dies are positioned in a press, which may be mechanical or hydraulic, which alternately opens and closes the dies to both stamp the component and enable feeding of the incoming flat sheets and removal of the stamping. A process cycle, feed-stamp-remove, for a large automotive stamping, such as a decklid, may be about 6-10 seconds. It should be noted that the stamping operation only imparts the intended three-dimensional shape to the intended part. Further operations, such as trimming to remove excess material, or punching to create openings are generally required to generate a finished part or component ready for assembly into a vehicle.
In the course of forming the component, the incoming sheet metal is deformed by an amount necessary to form the depressions, radii etc. required by the part design. The extent of the required deformation may be related to the geometry of individual features in the component, such as the depth of depression or the sharpness of a radius, or, in complex parts, by the interplay of the deformations required to produce a plurality of features. An attempt to form such a complex part may result in the sheet metal fracturing, splitting or tearing if the required deformation exceeds the capabilities of the sheet metal.
Sheets of lesser ductility are generally less formable and more prone to result in stampings with tears and splits than are sheets of greater ductility. For example, the forming severity of bends may be correlated with the R/T ratio for such bends, where R is the radius of the bend and T is the thickness of the sheet. Gentle curvature bends, such as 20T or greater, may be formed without undue difficulty. Often however, it is desired to form much sharper or ‘crisper’ features in a panel which may require 1T to 8T bends. Such small radius bends present more challenges to forming a part, particularly in materials of lesser formability. This may produce a disconnect between the desired complexity of the component and the ductility required of the incoming sheet metal to successfully stamp the component. Historically, this situation has been resolved by using sheet metal in its most soft, fully annealed condition, since this material will exhibit the greatest ductility. Of course, in consequence, the stamped component will be of minimum strength.
With the ongoing need for increased vehicle fuel economy, there is continuing interest in using higher strength-to-weight ratio materials. One effective approach to transition to higher strength-to-weight materials is to substitute materials of lesser density for materials of higher density, e.g. aluminum alloys for steel, and magnesium alloys for aluminum alloys. However after the initial benefit resulting from such substitution has been obtained, further improvement can only be achieved by increasing the strength, particularly the yield strength, of the alloys. As noted above, increased strength is commonly associated with reduced ductility so that improving the strength of an alloy can reduce its formability and so render it less suitable for stamping parts with complex features.
This reduction in formability with alloy strength is generally observed, but is particularly evident when strengthening results from work-hardening arising from cold-forming, that is, plastic deformation conducted ‘cold’, or for aluminum and magnesium alloys, at or about ambient temperature or 20-25° C.
There is thus a need for forming complex parts of higher strength materials.